1. Field of the Invention
The present invention relates to variable bit rate digital circuit multiplication equipment for carrying out tandem relay of speech.
2. Description of Related Art
Long-distance telecommunications including international telecommunications and others utilize DCME (Digital Circuit Multiplication Equipment) for reducing communication cost.
DCME is a device for efficiently transmitting voice-band data such as telephone speech, facsimile signals and data modem signals by combining a DSI (Digital Speech Interpolation) technique that transmits only speech activity with a low bit rate speech coding technique. In particular, variable bit rate DCME is a device that can vary its coding bit rate of the telephone speech in response to the load condition on a bearer line (transmission line).
FIG. 10 is a block diagram showing a configuration of a conventional variable bit rate DCME. In FIG. 10, the reference numeral 1 designates a speech activity decision section for receiving a PCM signal and for making a decision as to whether the input signal to each trunk channel is in speech active state or not; 2 designates a signal discriminating section for receiving the PCM signal and for identifying whether the input signal to each trunk channel is telephone speech or a data signal like a facsimile signal; 3 designates a speech coding section for encoding the PCM signal and outputting a coded speech signal; 4 designates an assignment controller for assigning transmission bit rate of each trunk channel to a bearer line in accordance with the decision result of the speech activity decision section 1 and the identification result of the signal discriminating section 2; 5 designates a message generator for generating an assignment message in accordance with the assignment result of the assignment controller 4; and 6 designates a multiplexer for multiplexing the coded speech signals of the individual trunk channels output from the speech coding section 3 in accordance with the assignment result by the assignment controller 4, and for multiplexing the assignment messages generated by the message generator 5, outputting the multiplexed signal to the bearer line.
The reference numeral 7 designates a demultiplexer that demultiplexes a signal from the bearer line including multiplexed coded speech signals and assignment messages, and supplies the assignment messages to a message decoder 8 and the coded speech signals to a speech decoder 9; 8 designates the message decoder that decodes each assignment message provided from the demultiplexer 7, and supplies the demultiplexer 7 with the decoded result and the speech decoder 9 with the assignment information on each trunk channel and with coding bit rate information; and 9 designates the speech decoder that decodes each coded speech signal provided from the demultiplexer 7 in accordance with the assignment information and coding bit rate information supplied from the message decoder 8, and sends a resultant PCM signal to each trunk (circuit switch) side channel.
In FIG. 10, the left-hand side is the trunk (circuit switch) side where the telephone speech/voice-band data of a plurality of channels are input or output in a 64-kbit/s PCM (Pulse Code Modulation) scheme. The right-hand side is a bearer (transmission line) side where the low bit rate coded telephone speech/voice-band data (coded speech signals) are transmitted or received.
Here, for convenience-in explanation, it is assumed that the trunk side has a capacity for transferring 600-channel 64-kbit/s telephone speech/voice-band data, and that the bearer side has a line capacity of 2 Mbits/s. It is further assumed in the following description that as the low bit rate speech coding bit rate, the telephone speech transmission uses 8 kbits/s or 6.4 kbits/s, whereas the voice-band data signal transmission utilizes 40 kbits/s.
Next, the operation of the conventional DCME will be described.
The 600-channel 64-kbits/s PCM signals input from the trunk side are supplied to the speech activity decision section 1, signal discriminating section 2 and speech coding section 3.
The speech activity decision section 1 decides the speech activity/silence of each trunk channel, and supplies the decision result to the assignment controller 4.
The signal discriminating section 2 decides as to whether the input signal to each trunk channel is the telephone speech or the data signal like a facsimile signal, and supplies the discrimination result to the assignment controller 4.
Receiving the decision result and discrimination result from the speech activity decision section 1 and signal discriminating section 2, the assignment controller 4 decides a bit rate assigned to each trunk channel on the bearer line in accordance with the decision result and discrimination result, and supplies the assignment result to the speech coding section 3, message generator 5 and multiplexer 6.
In the assignment to the bearer line, speech activity trunk channels are assigned to the bearer line first. In this case, trunk channels decided as transferring data signals are assigned 40 kbits/s per channel, whereas trunk channels decided as transferring telephone speech are each assigned 8 kbits/s or 6.4 kbits/s.
The coding bit rate is changed depending on the signal types. This is because the information compression principle of the low bit rate speech coding is based on reducing the redundancy of the speech signals by utilizing that redundancy, and hence high-degree compression is possible for the telephone speech, but not for the voice-band data like facsimile signals.
The telephone speech is assigned one of the two bit rates. It is usually assigned 8 kbits/s on the bearer line, which is reduced to 6.4 kbits/s when the bearer line becomes congested to enable new assignment.
As for a 32 kbit/s transmission line, for example, although it is occupied by four 8 kbit/s channels, it can provide five channels for 6.4 kbits/s.
The speech coding section 3 includes 600-channel speech encoders. Referring to coding bit rate information provided from the assignment controller 4 as the assignment result, the speech coding section 3 encodes the input signal from each trunk channel at a bit rate of 8 kbits/s or 6.4 kbits/s when it is the telephone speech, and at a bit rate of 40 kbits/s when it is the voice-band data, and supplies the coded speech signals to the multiplexer 6.
On the other hand, the message generator 5 generates the assignment message to be transferred to party equipment in accordance with the assignment result of the assignment controller 4.
An example of the assignment message will now be described with reference to FIG. 11 that shows a structure of a frame (DCME frame) the DCME outputs to the bearer line. In this example, there are 248 bearer channels (BCs) for transmitting the coded speech signals and a message channel for transmitting the assignment message on the bearer line.
Each BC has a capacity of 8 kbits/s so that 248-channel 8-kbit/s coded speech signals are transmitted at the maximum. A 40-kbit/s coded speech signal is transmitted using 5-channel BCs.
Incidentally, the DCME frame length is usually set at an integer multiple of an 8-kbit/s speech coding frame length and a 40-kbit/s speech coding frame length. For example, when the 8-kbit/s speech coding frame length is 10 ms and the 40-kbit/s speech coding frame length is 2.5 ms, the DCME frame length is preferably set at 10 ms.
In the present specification, the DCME frame length is assumed to be 10 ms in the following description (thus, the number of bits in each BC is 10 ms×8000=0.01 s×8000=80 bits). The message channel can transmit four messages, each of which consists of a pair of a trunk channel number (TC number) and a bearer channel number (BC number). For example, when the trunk channel “5” is newly connected to the bearer channel 3, a message TC number=5 and BC number=3 is transmitted.
Usually, the TC number=0 indicates disconnection. For example, to disconnect the trunk connected to the BC50, the message TC number=0 and BC number=50 is transmitted.
Thus, the assignment message is for transmitting to the party equipment the information about how each trunk channel is assigned to the bearer line. To save the message channel capacity, only information about a change in the assignment is formed as a message. Accordingly, when many changes take place as when many trunk channels simultaneously shift from a silent to speech activity state, some channels may have to wait until they are assigned to the bearer line.
In accordance with the assignment result to the bearer line by the assignment controller 4, the multiplexer 6 multiplexes the coded speech signals from the trunk channels output from the speech coding section 3, along with the assignment message output from the message generator 5, and outputs the multiplexed signal to the bearer line.
Next, the operation on the receiving side will be described.
The demultiplexer 7, receiving a signal including the coded speech signals and the assignment message multiplexed from the bearer line, demultiplexes them, and supplies the assignment message to the message decoder 8 and the coded speech signals to the speech decoder 9.
To demultiplex the coded speech signals, the demultiplexer 7 refers to the decoding result of the assignment message by the message decoder 8.
The mess age decoder 8, receiving the assignment message from the demultiplexer 7, decodes it and supplies its result to the demultiplexer 7. It also supplies the speech decoder 9 with the assignment information on the trunk channels and the coding bit rate information.
The speech decoder 9, receiving the assignment information and coding bit rate information from the message decoder 8, decodes the coded speech signals output from the demultiplexer 7 with reference to the information, and outputs PCM signals to the trunk side channels.
As described above, the DCME carries out low bit rate coding of the 64-kbit/s PCM signal sent via each trunk channel to an 8-kbit/s, 6.4-kbit/s or 40-kbit/s signal, and transmits the speech activity signals in precedence. Accordingly, it can transmit the telephone speech or facsimile signal efficiently.
Next, let us consider a network configuration as shown in FIG. 12, where such DCMEs are installed at three sites.
During communications between a telephone 110 and a telephone 111, the speech signal sent from the telephone 110 undergoes low bit rate coding by the DCME 100, and is decoded by the DCME 101 to a PCM signal. The PCM signal is transferred to a DCME 102 via a circuit switch 106. The DCME 102 carries out the low bit rate coding of the signal, and transmits it to a DCME 103. The DCME 103 decodes the low bit rate coded signal to a PCM signal, and sends it to the telephone 111. Thus, the network configuration as shown FIG. 12 that employs the DCMEs repeats the low bit rate coding and decoding twice, bringing about speech quality degradation.
To avoid such a problem, a technique called tandem passthrough is actually used in such fields as speech ATM communications.
FIG. 13 is a block diagram showing a configuration of a voice over ATM transmission system with the tandem passthrough function, which is disclosed in Japanese patent application laid-open No. 10-190667. In FIG. 13, the same reference numerals designate the same or like portions to those of FIG. 10, and hence the description thereof is omitted here.
In FIG. 13, the reference numeral 10 designates a cell disassembly section for disassembling ATM cells supplied from the bearer line side and outputting them; 11 designates a pseudo-speech signal generator that converts the 8-kbit/s and 40-kbit/s coded speech signal into a 64-kbit/s pseudo-speech signal that can be handled by the tandem circuit switch without decoding them (for example, the 8-kbit/s coded speech signal is converted into a pseudo 64-kbit/s signal by adding 56-kbit/s dummy data), and that outputs the pseudo-speech signal; and 12 designates a second comfort noise generator for generating comfort noise corresponding to background noise during idle state.
The reference numeral 13 designates a first pattern inserting section for inserting a first pattern signal that causes a party voice over ATM transmission system at the relay to identify that it is a tandem connection; 14 designates a selector for selecting and outputting either the pseudo-speech signal output from the pseudo-speech signal generator 11 or the comfort noise output from the second comfort noise generator 12; 15 designates a second pattern inserting section for inserting a second pattern signal that causes the party ATM system at the relay to identify that it is in the tandem switching state by detecting the second pattern signal; and 16 designates a selector for selecting and outputting either the output signal from the first pattern inserting section 13 or the output signal from the second pattern inserting section 15.
The reference numeral 17 designates a first pattern detector for detecting the first pattern signal sent from the party ATM system at the relay; 18 designates a second pattern detector for detecting the second pattern signal sent from the party ATM system at the relay; 19 designates a transmission bit rate restorer for converting the pseudo-speech signal sent from the circuit switch side into the coded speech signal with original coding bit rate by deleting the 56-kbit/s dummy data from the pseudo-speech signal; 20 designates a selector for selecting and outputting either the coded speech signal output from the speech coding section 3 or the codedspeech signal output from the transmission bit rate restorer 19; 21 designates a first comfort noise generator for generating low bit rate coded comfort noise corresponding to background noise in the idle state; 22 designates a selector for selecting and outputting either the low bit rate coded comfort noise output from the first comfort noise generator 21 or the coded speech signal output from the selector 20; and 23 designates a cell assembly section for assembling the coded speech signal into ATM cells and for outputting the cells.
The operation of the conventional ATM system as shown in FIG. 13 will now be described assuming that it is utilized in place of the DCME 100, DCME 101, DCME 102 and DCME 103 as shown in FIG. 12.
First, the operation of the voice over ATM transmission system used in place of the DCME 102 will be described when communication is conducted between the telephones 112 and 113 in FIG. 12 (in the case of non-tandem connection).
It is assumed in the initial state that the selector 14 selects the output of the pseudo-speech signal generator 11, the selector 16 selects the output of the first pattern inserting section 13, the selector 20 selects the output of the speech coding section 3, and the selector 22 selects the output of the selector 20, as shown in FIG. 13.
When the tandem circuit switch does not establish a tandem connection, since neither the first pattern detector 17 nor the second pattern detector 18 detects the first pattern signal or the second pattern signal from the output signal of sent from the trunk side, they output a signal indicating a non-detection state. Thus, the selectors 20, 22, 14 and 16 maintain their initial states.
Thus, the speech signal path on the transmitting side passes through the speech coding section 3, selector 20, selector 22 and cell assembly section 23, whereas the speech signal path on the receiving side passes through the cell disassembly section 10, speech decoder 9, first pattern inserting section 13 and selector 16 so that normal speech coding and decoding are carried out.
In this case, on the path on the receiving side, the first pattern inserting section 13 inserts the first pattern into the PCM signal output from the speech decoder 9.
The PCM signal output from the speech decoder 9 is a signal obtained by sampling a speech signal waveform at every 125 microseconds and by quantizing the amplitude of the sampled waveform into 8-bit data. Thus, it becomes a 64-kbit/s signal because 8÷125 microseconds=8÷0.000125=64000.
To minimize the degradation in the speech quality due to the first pattern insertion, the first pattern inserting section 13 carries out bit steal of only the LSB (Least Significant Bit) of an 8-bit quantized value at every several sampling interval for the PCM signal, thereby embedding a specified pattern. Thus, the first pattern insertion can implement communications without adding any substantial effect on the quality of the original PCM speech signal waveform. The operation of the voice over ATM transmission system placed at the position of the DCME 103, which is connected via the bearer line to the voice over ATM transmission system at the site of the DCME 102, is identical to that of the system placed at the site of the DCME 102.
Next, the operation of the voice over ATM transmission systems placed at the sites of the DCMEs 101 and 102 will be described when the tandem connection is established in the tandem circuit switch, that is, when the communication is carried out between the telephones 110 and 111 in FIG. 12.
When the voice over ATM transmission systems 60B and 60C corresponding to the DCMEs 101 and 102 are connected via the circuit switch 106 as shown in FIG. 14, the first pattern detector 17 of the voice over ATM transmission system 60B detects the first pattern inserted by the first pattern inserting section 13 of the voice over ATM transmission system 60C, and likewise the first pattern detector 17 of the voice over ATM transmission system 60C detects the first pattern inserted by the first pattern inserting section 13 of the voice over ATM transmission system 60B at the initial stage.
Thus, each of the voice over ATM transmission systems 60B and 60C changes its state such that the selector 16 selects the output of the second pattern inserting section 15, the selector 14 selects the output of the second comfort noise generator 12 and the selector 22 selects the output of the first comfort noise generator 21.
In each of the voice over ATM transmission systems 60B and 60C in this state, the signal path on the receiving side passes through the second comfort noise generator 12, selector 14, second pattern inserting section 15 and selector 16, whereas the signal path on the transmitting side passes through the first comfort noise generator 21, selector 22 and cell assembly section 23.
The second comfort noise generator 12 outputs 64-kbit/s PCM comfort noise. The second pattern inserting section 15 inserts the second pattern into the comfort noise (PCM signal) output from the second comfort noise generator 12. Specifically, the second pattern inserting section 15 carries out bit steal of only the second least significant bit of an 8-bit quantized value at every several sampling interval of the PCM signal to embed a specified pattern such that the second pattern can be distinguished from the first pattern and that the effect on the signal output from comfort noise generator 12 is minimized.
In this way, the voice over ATM transmission systems 60B and 60C each send a silent PCM signal including the second pattern to the circuit switch side. On the other hand, the first comfort noise generator 21 outputs a silent signal encoded at the 8 kbit/s bit rate or comfort noise. Accordingly, the voice over ATM transmission systems 60B and 60C send the silent signal or comfort noise to the bearer line side.
In the next stage, the voice over ATM transmission systems 60B and 60C each receive the silent PCM signal including the second pattern from the circuit switch side. Thus, the second pattern detector 18 detects the second pattern, and outputs a signal indicating its detection. In response to the signal, the selector 20 selects the output of the transmission bit rate restorer 19.
On the other hand, since the first pattern detector 17 cannot detect the first pattern, it outputs a signal indicating non-detection. In response to the signal, the current state is changed such that the selector 22 selects the output of the selector 20, and the selector 14 selects the output of the pseudo-speech signal generator 11.
As for the state of the selector 16, it maintains selecting the output of the second pattern inserting section 15 to be output. The pseudo-speech signal generator 11 generates the 64-kbit/s pseudo-speech signal by adding dummy data to the 8-kbit/s coded speech signal supplied from the cell disassembly section 10. The second pattern inserting section 15 inserts the second pattern to a part of the pseudo-speech signal. In this case, the pseudo-speech signal is assembled such that its part corrupted by inserting the second pattern becomes the dummy data. Thus, the 8-kbit/s coded speech signal is output without any problem.
The transmission bit rate restorer 19, receiving the pseudo-speech signal, extracts the 8-kbit/s coded speech signal and supplies it to the selector 20. The operation described above can implement the passthrough operation because the coded speech signal disassembled by the cell disassembly section 10 of the ATM transmission system 60B arrives at the cell assembly section 23 of the voice over ATM transmission system 60C, and reversely the coded speech signal disassembled by the cell disassembly section 10 of the ATM transmission system 60C arrives at the cell assembly section 23 of the voice over ATM transmission system 60B.
Applying the tandem passthrough function to the DCME as shown in FIG. 10 makes it possible for a link including the plurality of DCMEs to transmit telephone speech without degrading the speech quality.
With the foregoing configuration, the conventional digital circuit multiplication equipment has the following problems when the tandem passthrough function is applied to the variable bit rate DCME.
Let us consider a case, for example, where the telephone 110 communicates with the telephone 111 in FIG. 12, and the tandem passthrough operation is implemented by transmitting the speech signal through a trunk channel between the DCME 101 and the DCME 102. Here, the bearer line assignment from the DCME 100 to the DCME 101 can be changed depending on the speech activity/silence state, signal discrimination state and bearer load state detected by the DCME 100. For example, an increase in the bearer line load can change the speech coding bit rate of the speech signal sent from the telephone 110 from 8 kbits/s to 6.4 kbits/s. In this case, the DCME 101 can notify the DCME 102 of the change by embedding the speech activity/silence information and speech coding bit rate information into the pseudo-speech signal transmitted from the DCME 101 to the DCME 102.
It is also possible for the DCME 102 to determine the assignment of the trunk channel to the bearer line in accordance with the speech coding bit rate information and speech activity/silence information embedded into the pseudo-speech signal, and transmits information about the assignment to the DCME 103. However, it is not always possible for the DCME 102 to carry out the assignment to the bearer line as required by the speech coding bit rate information and speech activity/silence information embedded in the pseudo-speech signal, depending on the load condition of the bearer line to which the DCME 102 transmits the signal.
For example, if all the trunk channels connected to the bearer line are in speech activity state, and hence occupy the bearer line, even if a request arrives to change from 6.4 kbits/s to 8 kbits/s, the assignment change to the 8-kbit/s is detained, maintaining the 6.4-kbit/s state. Such a bit rate mismatch can also take place because the message number is limited. When such a mismatch takes place between the actual transmission bit rate of the coded speech signal and the assigned transmission bit rate on the bearer line, the speech decoder 9 cannot be provided with correct coding bit rate information, bringing about serious speech quality degradation.
As described above, implementing the tandem passthrough function by the variable bit rate DCME unavoidably involves a mismatch between the actual transmission bit rate of the coded speech signal and the assigned transmission bit rate on the bearer line, which presents a problem of degrading the speech quality seriously because the correct coding bit rate information cannot be supplied to the speech decoder.